Silver halide photography was well established prior to 1900 as the most desirable approach to obtaining photographic images, based primarily on exceptionally high levels of sensitivity and the capability of producing fine image detail with relatively low levels of noise, referred to in the art as granularity. Silver halide imaging compositions were originally thought to be emulsions, a term that is still used in the art, although it was soon appreciated that the radiation sensitive component of the emulsions were silver halide microcrystals, referred to as grains. From investigations of chloride, bromide and iodide ions in the grains, it was further appreciated that silver iodobromide grains exhibit superior speed-granularity relationships. For this reason, silver iodobromide emulsions are almost universally employed for camera-speed imaging applications. Silver iodobromide grains exhibit the face centered cubic crystal structure of silver bromide with iodide ions being present in minor amounts up to their solubility limit in silver bromide, typically less than 40 mole percent, based on total silver.
Initially the varied shapes of silver iodobromide grains were viewed as more a matter of scientific curiosity than practical significance. It was not until the early 1980's that photographic advantages, such as improved speed-granularity relationships, increased covering power both on an absolute basis and as a function of binder hardening, more rapid developability, increased thermal stability, increased separation of blue and minus blue imaging speeds, and improved image sharpness in both mono- and multi-emulsion layer formats, were realized to be attainable from silver iodobromide emulsions in which the majority of the total grain population based on grain projected area is accounted for by tabular grains exhibiting a high tabularity (T)--that is, greater than 25 when T is defined by the relationship: EQU T=ECD/t.sup.2 ( 1)
where
ECD is the effective circular diameter in .mu.m of the tabular grains and
t is the thickness in .mu.m of the tabular grains. Wilgus et al U.S. Pat. No. 4,434,226 and Kofron et al U.S. Pat. No. 4,439,520 are illustrative of early discoveries of high tabularity silver iodobromide emulsions and their advantageous photographic characteristics. More recently it has been recognized that thicker tabular grains, sometimes referred to as "slabular" grains, having aspect ratios (ECD/t) down to 2:1 and tabularities ranging upwardly from just to greater than 5 retain to at least some degree the advantages of high tabularity emulsions.
Still more recently it is has been recognized that further improvements in speed-granularity relationships can be realized by constructing tabular iodobromide grains with laminar strata differing in iodide concentrations. Sugimoto et al U.S. Pat. No. 4,665,012, Ohashi et al U.S. Pat. No. 4,835,095 and Saitou et al U.S. Pat. No. 4,945,037 are illustrative of silver iodobromide tabular grains emulsion containing laminar halide strata. In these emulsions advantages have been observed when at least 10 percent of the tabular iodobromide grains are formed of a host stratum having a relatively high iodide content while laminar strata interposed between the host stratum and the major surfaces of the tabular grains contain a relatively low iodide content. The laminar strata of the grains are typically of uniform composition.
It has long been recognized that metals can be incorporated in silver iodobromide emulsions as dopants to modify photographic properties. This is illustrated by Research Disclosure, Vol. 307, Dec. 1989, Item 308119, Section I.D. Research Disclosure is published by Kenneth Mason Publications, Ltd., Dudley Annex, 21a North Street, Emsworth, Hampshire PO10 7DQ, England.
Marchetti et al U.S. Pat. No. 4,937,180 recognized that formation of silver iodobromide grains in the presence of a hexacoordination complex of rhenium, ruthenium, or osmium with at least four cyanide ligands would increase the stability of the emulsions and reduce low intensity reciprocity failure. Marchetti et al recognized that the cyanide ligands were incorporated in the grain structure.
Shiba et al U.S. Pat. No. 3,790,390, Ohkubo et al U.S. Pat. No. 3,890,154, and Habu et al U.S. Pat. No. 4,147,542 disclose emulsions particularly adapted to imaging with flash (less than 10.sup.-5 second) exposures. Polymethine cyanine and merocyanine dyes are disclosed having up to three methine groups joining their nuclei, with blue flash exposures being suggested with zero, one or two methine linking groups and green flash exposures being suggested with three methine linking groups. In addition to the dyes it is suggested to incorporate in the emulsions compounds of Group VIII metals--i.e., iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum. Iron compounds suggested for incorporation are ferrous sulfate, ferric chloride, potassium hexacyanoferrate (II) or (III), and ferricyanide. Shiba et al, Ohkubo et al, and Habu et al suggest incorporation of the iron compounds at any convenient stage from precipitation to coating, indicating that whether the iron is located within or exterior of the grains is inconsequential to the utility taught.